PROJECT SUMMARY The current KOMP2 workflow does not enable phenotype data to be captured if the null mutation results in dominant lethality and founder mice are not produced. This subset of haplo-essential genes is thought to comprise up to 15% of the genome and is highly enriched in human disease. Here we propose an efficient and low-cost pipeline to annotate the phenotypes of haplo-essential genes, providing key data for researchers and clinicians with an accelerated rate of phenotype discovery compared with our current pipelines.